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SFG coming of age Surface chemists embrace this lesserknown technique for analyzing biomedical polymers and other applications. James P. Smith and Vicki Hinson-Smith
S
um-frequency generation (SFG) is a nonlinear, secondorder optical technique that provides information about surfaces and interfaces. It is applicable to many interfaces accessible by light and is commonly used to obtain IR vibrational spectra of molecules at an interface. Unlike the common electron spectroscopy surface methods, SFG can be used under ambient conditions at air/liquid, air/solid, and liquid/ solid interfaces. In the past, the study of surfaces has frustrated many chemists. Surfaces can be exceedingly reactive and particularly thin; they can be quickly altered by oxidation and by adsorption. Often, the only uncorrupted surfaces available for study have been those prepared in an ultra-high vacuum. That’s like studying the indigenous butterflies of the South Pole—in actuality, you seldom find one. In the dirty, wet, and complex world of chemistry, surfaces are often mere interfaces between two ill-defined phases. Past studies of surface chemistry dealt more with the two phases than with the interface itself, but the physics of SFG dictates that the signal must be from the surface molecular layer, rather than in the adjacent bulk materials. Worldwide, ~20–30 research groups have active SFG programs, and >100 SFG spectrometers may be in use globally. It is difficult to determine the exact number because most systems are “homemade”, and large chemical companies probably use SFG but do not publish their work. Because of the small market, only one company, EKSPLA Ltd., manufactures and installs a turnkey SFG spectrometer. The business was founded in 1992 in Vilnius (Lithuania), a city known for its photonics cluster—an active community of >20 photonics companies and research institutes, including the Laser Research Center of the University of Vilnius, the Institute of Physics, and the Institute of Semiconductor Physics. EKSPLA specializes in the manufacture of customized laser systems for research and engineering laboratories, industry, and medicine. But EKSPLA hardly has a monopoly. According to Heather Allen, who is active in instrument development at Ohio State University, “Ninety percent of the people doing SFG are using © 2004 AMERICAN CHEMICAL SOCIETY
their own home-built version, and each is different in some way from the others.” Like so many SFG researchers, Allen puts together her own optics and builds spectrometers around commercial lasers. Table 1 provides descriptions of four representative configurations of SFG spectrometers now in use. These include two Nd:YAG laser-based spectrometers, one with a nanosecond pulse width and the commercially available EKSPLA picosecond spectrometer. The other two systems use Ti:sapphire lasers—one is a picosecond scanning instrument and the other is a femtosecond broadband spectrometer. When comparing the two common types of lasers, says Dennis Hore, a researcher in Geri Richmond’s laboratory at the University of Oregon, “The advantage of the Ti:sapphire system over a YAG system is stability.” YAG lasers have been used and improved for years. They are relatively easy to use, and stability problems are minimized. However, the Ti:sapphire laser systems are generally more stable, although they are also more complex and expensive. SFG systems use optical parametric amplifiers (OPAs) to generate the tunable IR required. An OPA is a common piece of equipment used in a variety of experiments, and no extraordinary modifications are required to make a SFG spectrometer. “Everyone buys their pump laser from one of several laser companies,” Hore says. A U G U S T 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y
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Table 1. Representative configurations of SFG spectrometers. Only the EKSPLA instrument is commercially available. Description
Nanosecond
Picosecond (YAG-based)
Picosecond (Ti:sapphire oscillator-based)
Femtosecond (a.k.a. broadband)
For more information
Geri Richmond’s lab, University of Oregon 541-346-4648 http://richmondscience.uoregon. edu
Altos, Inc. (distributor for EKSPLA1) 406-581-782 www.altos-inc.com
Geri Richmond’s lab, University of Oregon 541-46-4648 http://richmondscience.uoregon. edu
Heather Allen’s lab, Ohio State University 614-292-4707 www.chemistry.ohio-state.edu/ ~allen
Pump laser
Nd:YAG at 1064 nm
Nd:YAG at 1064 nm
Ti:sapphire at ~800 nm
Ti:sapphire at ~800 nm
Pulse width
4–8 ns
20–30 ps
~2 ps
Visible: ~2 ps; IR: 50–300 fs
Repetition rate
10–20 Hz
Spectral range
10–20 Hz –1
2800–4000 cm
–1 Continuous tuning 2800–4000 cm range
2
1 kHz –1
1000–4000 cm–1
2800–4000 cm
1000–3100 cm
2800–4000 cm–1
1000–2800 cm–1 or 1800–3100 cm–1 A few hundred wavenumbers
4 cm–1 (with monochromator)
15–25 cm–1 (without monochromator2)
~15 cm–1 (with monochromator)
Resolution
1 cm–1
Limitations
Limited pulse intensity; total inter- Wide-range scans may require nal reflection geometry required; several minutes longer mid-IR wavelengths not possible
Wide-range scans may require several minutes; more expensive than YAG-based systems
Continuous tuning range is less than that of scanning instruments
Advantages and special features
Narrow bandwidth; simple configuration
New quantum dot assembly replaces laser dyes; wide tuning range
Stability of Ti:sapphire oscillator; wide tuning range
Instantaneous acquisition of entire spectrum, allowing time-resolved studies
Varies, but typically >$200,000
≥$500,000
≥$500,000
Approximate cost Typically $100,000–$150,000 1
1 kHz –1
EKSPLA’s webpage is www.ekspla.com. A monochromator may also be used with this setup, increasing the resolution.
SFG basics In SFG studies, a pulsed, tunable IR laser beam is mixed at the interface under study with a visible laser beam to produce an SFG signal with a frequency equal to the sum of the IR and visible frequencies. The analytical spot size is ~500 µm. As the IR wavelength is scanned, active vibration modes of molecules at the interface contribute resonantly to the SFG signal. This leads to a vibrational spectrum of the molecules located at the interface. For example, if the IR laser scans wavelengths from 2.3 to 18 µm (4350–556 cm–1) while the visible wavelength is held at 532 nm, then the SFG signal range is 432–516 nm. These visible wavelengths are detected by standard photomultiplier or CCD techniques. Unlike FTIR spectroscopy, the bulk materials do not contribute to the SFG signal. That’s because the frequency mixing occurs only at the interface. Nonlinear theory shows that the second-order nonlinear effects—such as second harmonic generation, SFG, and difference frequency generation—occur at an interface. Symmetry arguments rule that the second-order, nonlinear polarization in media with inversion symmetry (as found in bulk materials) is always equal to zero. This symmetry is broken at the interface. The SFG spectrometer detects vibration modes indicative of specific groups of atoms within molecules. It is also possible to 288 A
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obtain information about the relative orientation of different groups within the same molecule by changing the polarization of the input beams and analyzing the SFG signal polarization.
Instrumentation The first SFG surface spectroscopy was performed in 1986 by Y. R. Shen’s group at the Lawrence Berkeley National Laboratory. After Shen and Somorjai in the department of chemistry at the University of California recognized the potential of the nonlinear technique for surface characterization, they showed how the new technique could be applied to study systems such as metal surfaces, surfactant monolayers, and catalytic interfaces. Although sum-frequency and other nonlinear effects were studied in the 1960s, the development of SFG for surface spectroscopy was not possible without later innovations in high-energy, fast-pulse lasers. Hore explains the tradeoff between the IR pulse width and the minimum uncertainty bandwidth of the pulse. “The shorter the pulse width, the larger the spectral bandwidth of the IR pulse. A nanosecond laser pulse is inherently narrow[-bandwidth]. A picosecond pulse is broader, but the pulses are energetic enough to allow air/liquid interfaces to be studied, and are energetic enough to allow mid-IR generation—accessing more vibrational modes.”
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Pola
Ti:sapphire femtosecOne of the two (a) ond laser. This system basic designs for SFG has a 1-kHz repetisystems uses femtotion rate and good second pulses, which S/N for a scanning have a kilohertz repeinstrument. tition rate and very broad bandwidth— hundreds of waveFrom research to 160 numbers. But rather mainstream 120 than this becoming a SFG is
